The need for aseptic transfer, storage and processing of various biological liquids is well recognized. While this need is a practical consideration in such operations as microbiological fermentation and lyophilization, it takes on even greater importance in hematological operations. Thus for example, the various operations involved in blood transfusions necessitate the utmost care in the maintenance of aseptic conditions. The chances of contamination are especially great in indirect transfusion since the requisite transfer of the blood in the course of processing, freezing and storage involves a number of individual manipulations, each of which presents an opportunity for contamination. The present limitations on the permissible time during which previously frozen blood may be used are largely based on these contamination possibilities inherent in an open system.
In a closed system, the blood is transferred directly from the donor into a closed, aseptic system capable of performing the requisite processing steps. The blood is subjected to these various steps in different portions of the system without being removed at any time and without the introduction into the system of any other materials. While blood which is processed in a closed system can be safely maintained for a long period of time, the operation is so cumbersome and expensive when applied to frozen-thawed blood products that it is not presently feasible to employ.
In an open system, the various operations are conducted separately in different containers. Because of the inherent possibility of contamination in the course of transfer from one container to another and in the addition of processing materials, blood frozen and processed in an open system has a much shorter permissible period of use.
The actual freezing of the blood in the open system is generally performed in one of two ways. In the so-called "high glycerol content" technique, the blood is mixed with a quantity of glycerol to minimize cell damage and subjected to temperatures of -80.degree. to -90.degree. C. When the blood is ready for use, it is thawed, as for example through immersion in warm water, and then treated with an osmotic gradient such as saline solution to draw out the glycerol. In the second method, the so-called "low glycerol content" technique, the blood is rapidly frozen in the liquid phase of liquid nitrogen, that is at -196.degree. C. Although glycerol is also added in this technique prior to the freezing, the amount which is added is considerably less than in the first technique. When the blood is to be used, it is thawed, again simply by immersion in warm water, and again treated with an osmotic gradient to remove the glycerol.
Originally, the container for the blood was fabricated out of stainless steel in order to withstand the stresses encountered on a structure upon plunging it into liquid nitrogen. These containers were invariably rectangular canisters having a thickness no greater than about three-eighths of an inch in order to incur the rapid and uniform freezing of the canister's contents. More recently, the use of stainless steel canisters has been largely replaced through the introduction of special plastic containers. These are generally constructed out of polyvinyl chloride and supported in a rigid cassette. To minimize the dangers of contamination, current regulations and practice precludes the use of any plasticizer in the formulation of the PVC. While the plastic containers are an improvement over the stainless steel containers, experience has shown that a failure rate of about 2 to 3 percent can be expected. These failures, which probably occur in the course of freezing but which cannot be detected until thawing, constitute a very serious disadvantage to the use of plastic containers for the freezing of blood. Moreover, plastic containers are not reuseable and thus constitute an expensive if not wasteful luxury.
Finally, in any open system, whether the container employed is a stainless steel canister or a plastic bag, the possibility of contamination in the course of transfer and the resultant limitations on the time in which the blood may be safely used are highly undesirable. It is clear from the foregoing that a definite need exists for a system which eliminates the possibility of outside contamination yet permits the flexibility of the open system of blood freezing and at the same time permits the minimization of loss through container failure and the reuse of the containers.